This invention relates in general to plastic bottles for beverages and more particularly to an improved self supporting base for such bottles which provides increased strength to resist bottom roll out due to internal carbonation pressures.
A major difficulty with the use of plastic beverage bottles for carbonated beverages is the strength of the bottom of the bottle. Due to internal carbonation pressures which can be as high as 100 psi, plastic bottles have a tendency to bulge outward at the bottom creating what is referred to as a "rocker" which will rock back and forth when standing and/or possibly tip over. In addition, as the bottom of the bottle bulges out, the volume of the bottle increases, thereby lowering the fill line such that customers are led to believe the bottle is not properly filled or sealed.
One solution is to provide a bottle having a hemispherical bottom and attach a second plastic piece which comprises a support stand for the bottle. This solution however, adds considerably to the weight and cost of the bottle. Several bottles have been developed which include a self supporting base molded into the bottle. One way to manufacture a self supporting bottle which resists rollout is to increase the amount of plastic material in the base. The amount of material necessary to provide sufficient strength, however, results in a prohibitedly expensive bottle.
Other bottles have been developed which incorporate a number of features into the bottom to prevent roll out. One such bottle is illustrated in U.S. Pat. No. 3,727,783 which includes, among other features, an axially aligned re-entrant cylinder about the center of the bottle base. This re-entrant however, is difficult to blow mold in small bottles with a volume of one liter or less.
Another bottle is illustrated in U.S. Pat. No. 3,598,270 which illustrates what is known as a petaloid design. The petaloid design is also difficult to use for small volume bottles under one liter because the petaloid feet are cumbersome to blow into such small diameter bottles. The petaloid design also requires more material, adding excessive weight to the bottle. Additionally, the diameter of the contact points is relatively small, limiting the stability of the bottle.
Another bottle design is illustrated in U.S. Pat. No. 4,261,948 which has become known as the "supa" bottle. The "supa" bottle is similar in design to a champagne bottle having an inward depression or cone at the bottom of the bottle. The "supa" bottle includes a number of radially outward reinforcing ribs molded into the inner surface of the cone, thereby increasing its strength and eliminating roll out of the cone section. These ribs are formed by molding longitudinal ribs into the end cap area of the injection molded preform. During the blow molding process, these ribs act to reduce the amount of material stretching in the bottle base. As a result of reduced stretching, the wall thickness of the base is greater than in bottles without the reinforcing ribs. The "supa" bottle is more difficult to blow mold because the stretch of the ribbing must be precisely controlled. Additionally, with the "supa" bottles, the wall thickness of the contact area is difficult to control. Contact areas with thinner walls will creep more when the bottle is pressurized than areas with thicker walls, this results in a bottle which will not stand perpendicularly.
Accordingly, it is an object of this invention to provide a small volume plastic bottle in which the process parameters are less restrictive than with the "supa" bottle.
It is another object of this invention to reduce the weight of the bottle and distribute the thermoplastic material in a more equitable manner throughout the bottle.
A further object of this invention is to provide a bottle having improved perpendicularity.
A still further object of the invention is to provide a bottle having improved stability.
It is an advantage of this invention that the bottle weight can be reduced resulting in a cost savings of the material used.
It is a further advantage that the more evenly distributed material will increase the stability of the bottle and the shelf life of the carbonated product within the bottle.